A. Field of the Invention
The present invention is related to oil and gas well drilling operations and more particularly to determining the condition to update the bit depth value based on travelling block motion above a drilling rig floor and motion of the drill string.
B. Description of the prior art
Oil and gas wells are drilled by means of drilling rigs. The drilling rig generally consists of a mast or derrick that is mounted over a rig floor and a substructure. The drill string is moved vertically in the rig by a block and tackle arrangement suspended from the mast, which includes a crown block mounted near the top of the mast or derrick and a travelling block that is movable with respect to the crown block by a cable. The cable is strung between the crown block and the travelling block and the end of the cable is carried by a drawworks drum. The change in the block height when the drawworks drum is rotated is approximately equal to the amount of cable paid out or taken in by the drawworks divided by the number of lines strung between the crown block and the travelling block.
An important parameter during well drilling is the position of the travelling block above the rig floor. This position can be differentiated with respect to time to indicate the velocity of the drill string during tripping and the rate of penetration during drilling. The value of block height may also be accumulated to indicate the depth of the drill bit.
Motion of the traveling block may be associated with drilling operations in which the drill string does not move. For example, in tripping the string, the traveling block may transition through several up and down motions for each connection or disconnection of pipe sections in the drill string. Consequently, if bit depth measurement employed only position of the traveling block while ignoring actual motion of the drill string, significant errors could result. Therefore, only changes in block height, which accompany actual drill string motion, should be incorporated into the bit depth and bit rate calculations.
An example of a system developed for calculating the block height is described in a patent application, serial no. 07/762,745, titled "METHOD AND APPARATUS FOR DETERMINING THE HEIGHT OF A TRAVELLING BLOCK ABOVE A RIG FLOOR."
A detection of hook load has traditionally been used to determine if the drill pipe is connected to the travelling block. However, it becomes very difficult to determine the presence of the drill pipe if the pipe is very short and light. Furthermore, the error of calculation increases when the drill pipe is farther away from the rig floor. The sensitivity of the hook load transducer becomes less capable of detecting changes due to the large weight span of the pipe. The present invention avoids such insensitivities and is unaffected by the weight restrictions and ambient conditions.